In the production of a liquid crystal devices, including liquid crystal displays (LCD), a film, such as a polyimide film, is applied to the LCD glass. The film is then made to be anisotropic either by mechanically rubbing the film, or by the relatively newer technique of exposing the film to polarized light. Once the film has been made anisotropic, liquid crystal (LC) molecules will tend to align themselves to the anisotropic axis of the film. Further, the magnitude of the film anisotropy relates to the anchoring energy between the LC molecule and the film, and in the case of rubbed PI, dictates the pre-tilt angle of the LC molecule. The anisotropic magnitude and orientation of the optically anisotropic film in a liquid crystal device are major factors determining the performance of the liquid crystal device, and manufacturers need a means for measuring these parameters.
The optical anisotropy is typically the difference between the ordinary and extraordinary refractive indices, Δn=no−ne, of the film. Because of this anisotropy, the film will exhibit the polarization property of phase retardation or retardance. For an optical beam passing through an anisotropic material, the phase retardance experienced by the beam is δ=Δn×t, where t is the thickness of the material.
The optical anisotropy can also be the difference between the ordinary and extraordinary extinction coefficients, or imaginary components of the refractive index, Δk=ko−ke. In this case, the anisotropy will exhibit the polarization property of dichroism or diattenuation, where the transmittance, or reflectance, of an optical beam varies depending on the polarization state of the optical beam.
In modern liquid crystal devices, the PI film thickness tends to be on the order of 50 nm-100 nm. For rubbed PI, the anisotropy tends to be around Δn≈0.005. And so the retardance of this rubbed PI film would by on the order of 0.25 nm-0.5 nm. For some types of photo-aligned PI, the anisotropy is considerably larger, perhaps Δn≈0.05. In this case, the largest retardance of the anisotropic film that would be expected is around 5 nm.
Accurately measuring retardance magnitude and retardance orientation for samples with retardance in the range 0.25 nm-5 nm is conventionally difficult without proper equipment, however several commercially available measurement systems (e.g., AxoScan™ from Axometrics Inc. of Huntsville, Ala.) can make this measurement with sufficient accuracy. But a problem with making this measurement is that the glass substrate upon which the PI layer is applied will also exhibit retardance due to small stresses within the glass caused during manufacturing. Although the retardance of the glass is quite small, so is the retardance of the anisotropic PI layer. And it is difficult to separate these two sources of retardance in the measurements. As such, simple measurement techniques that only measure the retardance of a sample in transmission have not proven adequate for this application.
Ellipsometry is an optical technique for measuring the thickness and refractive index of thin films based on measuring how a film changes the polarization state of light when the light is reflected from the surface of the sample. Generalized Ellipsometry (GE) is an extension of standard ellipsometry that allows testing anisotropic samples. The GE method is accurate and precise. However, because this method is time-consuming, it is not always practical to use GE measurement in a production environment. An additional difficulty of the GE method is that it becomes increasingly difficult as a measured sample gains in complexity. If a sample has many layers of thin films, additional wavelengths may be needed. Or if the films of the sample are patterned with features smaller than the measurement optical beam diameter, the GE technique may fail completely. Because of those difficulties, the GE method may not be suitable in cases where PI is deposited on the color filter (CF) glass, or on the thin film transistor (TFT) glass of modern high-resolution LCD displays, thus limiting the GE technique to measurement of PI on test glass.
Other efforts that have been made to characterize anisotropic PI films are described in literature discussed below. These techniques focus primarily on rubbed PI films, since rubbed PI films have been used in liquid crystal display industry for decades. However, these methods should also be applicable to measuring photo-aligned PI films also. These methods are described below.
The orientation and anisotropy of rubbed PI films have been studied by retardance measurement, infrared dichroism measurement, and surface second harmonic-generation (see N. A. J. M. van Aerle, et. al., “Effect of rubbing on the molecular orientation within polyimide orienting layers of liquid crystal displays,” J. Appl. Phys. 74(5), 3111-3120 (1993), which is incorporated by reference herein). Retardance from the substrate has to be taken into account in the retardance measurement, the infrared dichroism measurement is not sensitive enough due to a very thin film, and surface second harmonic generation requires a complicated measurement setup. Grazing-incidence X-ray scattering method is capable of studying anisotropic PI films in a more sensitive way (see M. F. Toney, et. al., “Near-surface alignment of polymers in rubbed films,” Nature 374(20), 709-711 (1995), which is incorporated by reference herein). Generalized ellipsometry method also has been used to study the magnitude and orientation of anisotropic PI films (see I. Hirosawa, “Method of characterizing rubbed polymide film for liquid crystal display devices using reflection ellipsometry,” Jpn. J. Appl. Phys. 35, 5873-5875 (1996); see also I. Hirosawa, “Relation between molecular orientation and rubbing strength observed by reflection ellipsometry,” Jpn. J. Appl. Phys. 36, 5192-5196 (1997), which are both incorporated by reference herein). Most recently, a Polarization-Conversion Guided Mode technique has been developed to quantify optical anisotropy as low as 10−5 for a 10 nm surface layer (see F. Yang, et. al., “Polarization-Conversion Guided Mode (PCGM) technique for exploring thin anisotropic surface layers,” Optics Express 15(18), 11234-11240 (2007); see also F. Yang, et. al., “Optical anisotropy and liquid-crystal alignment properties of rubbed polyimide layers,” Liquid Crystals 34(12), 1433-1441 (2007), which are both incorporated by reference herein). This method requires a prism coupler and refractive index matching fluid to contact the sample. Reflection anisotropy spectroscopy (B. F. Macdonald, et. al., “Reflection anisotropy spectroscopy: A probe of rubbed polyimide liquid crystal alignment layers,” J. Appl. Phys. 93(8), 4442-4446 (2003), which is incorporated by reference herein), a technique used to characterize electronic surface states in semiconductors at normal incidence, can be used to test the anisotropy properties of PI films on glass. Most of the above-mentioned methods are too difficult to implement or have insufficient accuracy for use in a production environment.
Another apparatus for measuring the magnitude and orientation of anisotropy has been described (JP Patent Publication No. 2008-76324 11-304645 JP, which is incorporated by reference herein). This technique uses rotating retarders and a half mirror for making measurements in retro-reflection at normal incidence.
It is known that the anisotropy orientation and the relative anisotropy magnitude of PI film on glass can be measured by illuminating a sample at an oblique angle with light polarized in the s plane (polarized parallel to the plane of incidence), and collecting the reflected beam through a polarizer orientated along the p plane (perpendicular to the plane of incidence), and observing the signal as the sample is rotated about its normal through 360 degrees (see Taiwan Patent 095102013, which is incorporated by reference herein). Applying a curve-fit to this signal vs. rotation data allows the anisotropy orientation and the relative anisotropy magnitude to be measured. The relative anisotropy magnitude refers to some measurement parameter that varies when the actual anisotropy magnitude varies. The relative value might not be well calibrated to real values, however this is adequate for monitoring variations during the production process.
The technique described in Taiwan Patent 095102013, which is incorporated by reference herein, is fast enough for use in production environments, and this technique is currently used in the LCD industry. This technique suffers at least one disadvantage when measurements are made on the active area of an LCD glass. The active area is the area of the glass with the patterned pixels, either the color filter (CF) or thin-film transistor (TFT) area. When measuring the CF or TFT active area, this technique requires that a microscopic measurement beam be focused onto a single pixel, and that the measurement apparatus be rotated precisely around this point so that the measurement beam remains in the center of the pixel being tested. As the pixel density of modern cell phones and tablet displays has increased to 300 pixel per inch and beyond, the requirement that the measurement point remain centered on the pixel during rotation significantly increases the system complexity and price. Accordingly, there is a desire for methods and systems to solve these and other related problems.